Service providers will generally try to resolve problems so customers will _ _ them to other people.
a. urge
c. recommend
b. advocate
d. inform

Service providers will generally try to resolve problems so customers will recommend them to other people. Recommend meaning to tell others about how good the service provider is and therefore receive more business. Urge does not work because that means to earnestly persuade someone to do something. Advocate can be a noun or a verb, as a noun it is someone who publicly supports or recommends; as a verb, it is to publicly recommend. While it is the second best answer, it sounds awkward to use it in this particular context. Finally, to inform means to give someone information.